In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, the ink receptive regions accept the ink and repel the water. The ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the materials upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the element is considered as positive-working. Conversely, if the non-exposed regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel ink.
Similarly, positive-working compositions can be used to form resist patterns in printed circuit board (PCB) production, thick-and-thin film circuits, resistors, capacitors, and inductors, multichip devices, integrated circuits, and active semiconductive devices.
“Laser direct imaging” methods (LDI) have been known that directly form an offset printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There has been considerable development in this field from more efficient lasers, improved imageable compositions and components thereof.
Thermally sensitive imageable elements can be classified as those that undergo chemical transformation(s) in response to, exposure to, or adsorption of, suitable amounts of thermal energy. The nature of thermally induced chemical transformation may be to ablate the imageable composition in the element, or to change its solubility in a particular developer, or to change the tackiness or hydrophilicity or hydrophobicity of the surface layer of the thermally sensitive layer. As such, thermal imaging can be used to expose predetermined regions of an imageable layer that can serve as a lithographic printing surface or resist pattern in PCB production.
Positive-working imageable compositions containing novolak or other phenolic polymeric binders and diazoquinone imaging components have been prevalent in the lithographic printing plate and photoresist industries for many years. Imageable compositions based on various phenolic resins and infrared radiation absorbing compounds are also well known.
A wide range of thermally-sensitive compositions that are useful in thermal recording materials are described in patent GB 1,245,924 (Brinckman), whereby the solubility of any given area of the imageable layer in a given solvent can be increased by the heating of the layer by indirect exposure to a short duration high intensity visible light and/or infrared radiation transmitted or reflected from the background areas of a graphic original located in contact with the recording material. Several systems are described that operate by many different mechanisms and use different developers ranging from water to chlorinated organic solvents. Included in the disclosed aqueous developable compositions are those that comprise a novolak type phenolic resin. Coated films of such resins are said to show increased solubility upon heating. The compositions may also contain heat-absorbing compounds such as carbon black or Milori Blue (C.I. Pigment Blue 27) to additionally color the images.
Thermally imageable, single and/or multi-layer elements are also described in WO 97/39894 (Hoare et al.), WO 98/42507 (West et al.), WO 99/11458 (Ngueng et al.), U.S. Pat. Nos. 5,840,467 (Kitatani), 6,060,217 (Ngueng et al.), 6,060,218 (Van Damme et al.), 6,110,646 (Urano et al.), 6,117,623 (Kawauchi), 6,143,464 (Kawauchi), 6,294,311 (Shimazu et al.), 6,352,812 (Shimazu et al.), 6,593,055 (Shimazu et al.), 6,352,811 (Patel et al.), 6,358,669 (Savariar-Hauck et al.), and 6,528,228 (Savariar-Hauck et al.), and U.S. Patent Application Publications 2002/0081522 (Miyake et al.) and 2004/0067432 A1 (Kitson et al.).
The industry has focused on the need to diminish the solubility of the exposed regions of phenolic binders (dissolution inhibitors) in the imageable layers before exposure and to enhance their solubility after exposure to suitable thermal energy (dissolution enhancers). WO 2004/081662 (Memetea et al.) describes the use of various phenolic polymers or poly(vinyl acetals) in combination with developability-enhancing compounds of an acidic nature in positive-working compositions and elements. Some particular poly(vinyl acetals) useful in this manner are described in U.S. Pat. Nos. 6,255,033 (Levanon et al.) and 6,541,181 (Levanon et al.).
Some useful poly(vinyl acetals) are described in U.S. Ser. No. 11/677,599 (filed Feb. 22, 2007 by Levanon et al.), EP 1,627,732A1 (Hatanaka et al.), and U.S. Published Patent Applications 2005/0214677 (Nagashima), 2005/0214678 (Nagashima), and 2006/0275698 (My T. Nguen)
Problem To Be Solved
Offset printing plates recently have been the subject of increasing performance demands with respect to resistance to solvents and common printing room chemicals. Printing plates encounter pressroom chemicals such as plate cleaning agents, blanket washing agents, and alcohol substitutes in the fountain solution. Particularly in printing processes using ultraviolet-curable inks, where rinsing agents with a high content of esters, ethers or ketones are used, the chemical resistance of conventional positive-working printing plates can be improved.
Imaged regions in such printing plates should be substantially insoluble in ultraviolet-curable inks and substantially insoluble in solvents, often glycol ethers, used to clean plates during or after a print run. Conventional quinone diazide/phenolic resin-based radiation-sensitive compositions are soluble in glycol ether solvents, and are disfavored for printing with ultraviolet-curable inks.
Another demand is that the imaged regions be substantially insoluble in the fountain solutions (or dampening liquids) that are used to wet the hydrophilic areas of the plates. Conventional fountain solutions are largely made up of water and a small amount of alcohol. More recently, such fountain solutions have been replaced, in some situations, with formulations comprising alternative additives in order to remove inflammable alcohol solvents from press room environments. Additives that have been used in this manner include surfactants and various non-volatile solvents that may be more aggressive towards the radiation-sensitive compositions. Conventional radiation-sensitive compositions are relatively susceptible to attack by replacement fountain solutions.
A need remains for positive-working, thermally imageable elements that have improved bakeable and improved resistant to press chemistries, such as lithographic inks, fountain solutions, and the solvents used in washes, such as UV washes. Bakeability is highly desirable because baking increases the press run length for the printing plates.